It's often said that dogs are man's best friend - and new research appears to prove it once again. In a new study, scientists from Queen's University Belfast reported that dogs can smell stress from human sweat and breath. Using sweat and breath samples from 36 participants, the team analysed their stress levels, blood pressure, and heart rate before and after they attempted a difficult maths problem. Four dogs were taught how to search a scent line-up and alert researchers to the correct sample.